fix(dk/cvr): reject numbers that start with 0

[kallehauge]

Problem

dk/cvr accepts 00000000 as a valid CVR number:

stdnum.DK.cvr.validate('00000000');
// { isValid: true, compact: '00000000', isIndividual: false, isCompany: true }

Its modulus 11 sum is 0, so the check digit matches. 00000000 is a common placeholder in VAT-number fields, so it ends up treated as a real company number.

Fix

No CVR number starts with 0. python-stdnum's dk/cvr refuses those with InvalidFormat (stdnum/dk/cvr.py#L56):

if not isdigits(number) or number[0] == '0':
    raise InvalidFormat()

This PR adds the same check.

Input	Before	After
00000000	valid	InvalidFormat
DK00000000	valid	InvalidFormat
01000004 (passes modulus 11)	valid	InvalidFormat
24256790 (Novo Nordisk A/S)	valid	valid
13585628	valid	valid

Tested

• src/dk/cvr.spec.ts: 3 new cases that fail before the change and pass after it, plus 2 showing a real company's CVR still passes (Novo Nordisk A/S, valid in the EU VIES service and active in the Danish CVR register).
• Full suite: TZ=UTC npx jest --runInBand passes, 200 suites / 1640 tests.
• npm run lint passes.